Mirroring repositories doesn't work: 2:Fetching remote upstream failed

Problem to solve

The behavior of Git changed in 2.20.0, which now causes mirroring to fail if tags have diverged:

* The rules used by "git push" and "git fetch" to determine if a ref
   can or cannot be updated were inconsistent; specifically, fetching
   to update existing tags were allowed even though tags are supposed
   to be unmoving anchoring points.  "git fetch" was taught to forbid
   updates to existing tags without the "--force" option.

We have received multiple reports related to Mirror repository functionality not working properly.

Further details

We currently have an option to force mirror changes from upstream, or allow branches to diverge. We should allow similar behavior for tags.

Proposal

Change Overwrite diverged branches button to Overwrite diverged branches and tags
if this button is checked, then we will use --force to overwrite tags if the tags have been modified in the remote

Links / references

Customer Reports:

https://gitlab.zendesk.com/agent/tickets/120847 (internal)
https://gitlab.zendesk.com/agent/tickets/120852 (internal)

Edited May 16, 2019 by James Ramsay (ex-GitLab)